The invention relates generally to the field of support racks and, more particularly, to racks used to support printed circuit boards.
Few products are sold by their manufacturer without some type of testing being conducted. Such testing may be as simple as manually ascertaining whether certain parts are securely affixedxe2x80x94or as complex as xe2x80x9cstress testing.xe2x80x9d In stress testing (or xe2x80x9cstress screeningxe2x80x9d as it is sometimes called), products exhibiting xe2x80x9cinfant mortalityxe2x80x9d fail outright during the test. Or as the result of such testing, a product may evidence the likelihood of early failure in the operating environment. And some aspects of stress testing involve testing to destruction while measuring certain performance parameters and/or the elapsed time to failure.
Stress testing is most frequently employed with respect to products used in demanding applications and for which exceptionally-high reliability is required. Examples include products used on ground-travelling military equipment and products (e.g., electronic and electromechanical products) used in aircraft of essentially all types.
Stress testing may be carried out in any of several different ways. One type of test regimen involves imposing rapid, extreme changes in temperature upon the product. As an example, a test chamber may be used to change the temperature of a product at the rate of, e.g., 10 degrees C. (about 18 degrees F.) per minute. Exemplary environmental test chambers used for such test regimens are disclosed in U.S. Pat. No. 5,072,177 (Liken et al.) and U.S. Pat. No. 5,191,282 (Liken et al.).
Another type of stress testing involves testing a product by subjecting it to vibrations of the type which might be encountered in actual product use. Vibration stresses on the order of ten times the force of gravity (xe2x80x9c10 Gsxe2x80x9d) are not uncommon. Vibration testing is carried out by mounting the product to be tested upon some sort of platform or table and then vibrating the table using a rotating eccentric or a linear vibrator. An exemplary vibrator-driven table apparatus used for such stress testing is disclosed in U.S. Pat. No. 5,804,732 (Wetzel et al.).
Yet another type of stress testing is known as highly accelerated stress testing (HAST) and involves subjecting products, e.g., printed circuit boards (PCBs), to vibrational stress forces as high as 50Gs. Such forces are applied randomly to the product along and in six axes of motion (as the industry describes it), i.e., along each of the linear X, Y and Z axes and in rotational axes about such linear axes. Such vibrational testing may be carried out alone or while the PCBs are also subjected to extreme thermal stress involving temperature changes at the rate of 60 degress C. (about 110 degrees F.) per minute. In the latter instance, the vibrator table with PCBs supported thereon is placed inside a chamber. Such chamber is configured with heating and refrigeration capabilities to permit extreme and rapid temperature changes inside the chamber simultaneously with vibration testing. A leading manufacturer of environmental and vibrational testing equipment is Thermotron Industries of Holland, Mich.
When configuring PCB support racks for such tests, there are number of design considerations. Among them are circulation of air around the PCBs (to facilitate the aforedescribed rapid changes in the temperature of the boards) and transfer of vibrational forces to the PCBs.
While prior art PCB support racks (such as, e.g., the carrier disclosed in U.S. Pat. No. 5,268,637 (Liken et al.) have been generally satisfactory for their intended purposes, they are not adequate for vibrational testing, especially the HAST regimen described above. A reason relates to transfer of vibrational forces from the vibrator table to the PCBs. The carrier shown in the aforementioned Liken et al. ""637 patent contemplates slide-in and plug-in board mounting. Such mounting does not adequately transfer vibrational forces.
An improved support rack for vibration and thermal testing of printed circuit boards would be a distinct advance in the art.
It is an object of the invention to provide a circuit board rack overcoming some of the problems and shortcomings of earlier racks.
Another object of the invention is to provide a circuit board support rack which may be used for highly accelerated stress testing.
Yet another object of the invention is to provide a circuit board support rack with rigidity and board clamping capability selected to cause transmission of the energy of vibration from a vibratory table to the boards undergoing test.
Another object of the invention is to provide a circuit board support rack which accommodates a range of sizes of printed circuit boards.
Still another object of the invention is to provide a circuit board support rack configured to permit ample air flow around and across the boards for thermal stress testing. How these and other objects are accomplished will become apparent from the following descriptions and from the drawings.
The invention involves a rack for supporting printed circuit boards during vibratory (and, optionally, thermal) testing which, often, involves testing the boards to destruction. The purpose of such testing is to ascertain how sturdily the boards are designed and manufactured and how much severe service they might withstand before failure.
The new circuit board support rack comprises a frame which is generally orthogonal in configuration and includes a pair of upright, longitudinally-spaced end panels having a pair of laterally-spaced lower rails extending therebetween. A board engagement platform is fixed with respect to the frame and is xe2x80x9cframedxe2x80x9d by the end panels and rails. Such platform includes plural edge engagement members extending laterally between the rails. Each such member has a linear engagement groove for receiving and securing the lower edge of a respective printed circuit board.
Spaced above the platform by some dimension and oriented generally parallel thereto is a board retention member. The dimension by which such member is spaced from the engagement platform may be selected by a first adjustment mechanism. In a specific embodiment, each end panel has two vertical rows of apertures. Such rows are spaced laterally from one another and the apertures defining a row are, preferably, spaced vertically from one another.
When the board retention member is located at the approximate desired dimension above the engagement platform (i.e., desired in view of the edge-to-edge measurement of the boards to be supported), respective shoulder screws are inserted through corresponding apertures in each of the four vertical rows of apertures and threaded snugly into the retention member. The apertures and shoulder screws coact with the frame and the retention member and comprise the first adjustment mechanism. Such mechanism permits adjusting, in small, predetermined increments, the dimension between the platform and the retention members Assuming the end panels, rails, engagement platform and retention member are tightly and securely affixed to one another, the rack will transmit vibration from the vibratory table to the circuit boards under test.
The rack also includes at least one second adjustment mechanism (and preferably a plurality thereof) on the retention member. Each such second adjustment mechanism is mounted for movement toward and away from the engagement platform while the platform-to-retention member dimension is held substantially constant. In a specific, highly preferred embodiment, the rack has at least first and second pluralities of second adjustment mechanisms. Most preferably, each plurality includes three laterally-spaced adjustment mechanisms.
Merely as an example, the first plurality of second adjustment mechanisms includes first, second and third adjustment mechanisms. Further, each of the latter mechanisms comprises a respective clamping screw (i.e., first, second and third clamping screws), each having its own locating pin (i.e., first, second and third locating pins). When the first or lower and second or upper edges of a printed circuit board are inserted between, respectively, a particular edge engagement member and the second adjustment mechanism (or plurality of such mechanisms) immediately thereabove, the locating pins of the individual mechanisms are oriented so that the notches therein engage the upper printed circuit board edge. The second adjustment mechanisms are tightened to securely hold the board for vibratory testing. When such mechanisms are tightened, compressive force is exerted on the boards by the engagement platform and the retention member. Such force might be considered to be exerted substantially parallel to or substantially coincident with a planar surface of the printed circuit board.
From the foregoing and, particularly, from the drawings and the detailed description which follows, certain dimensional relationships of a specific, preferred embodiment will be apparent. The first dimension, i.e., that dimension between the engagement platform and, say, the underside of the board retention member, may be selected in increments that are predetermined by the spacing between apertures of the first adjustment mechanism. It is fair to say that this configuration permits adjustment that is somewhat xe2x80x9ccoarse.xe2x80x9d
It is to be understood that the locating pins protrude below the underside of the retention member. Thus, the locating pins (or at least an exemplary pin) is spaced from the engagement platform by a second dimension which is somewhat less than the first dimension. The aforedescribed configuration of the second adjustment mechanism permits the second dimension to be selected in a continuum rather than in descrete increments. One might say that such configuration permits xe2x80x9cfinexe2x80x9d adjustment to securely clamp each of the boards to undergo test.
To effect such testing, the frame (and, more specifically, each of the rails of such frame) include plural openings therealong. The spacing between such openings is preferably selected to conform to the spacing of holes in the vibratory table. To secure the rack, boards and vibratory table to one another, bolts are extended through such openings and threaded into the holes in the table.
Further details of the invention are set forth in the following detailed description and in the drawings.